The present invention relates generally to an apparatus and method for providing a feedback signal repesentative of the current through a load driven by a current amplifier. The present invention has particular application in constructing a feedback signal representative of the torque producing current through a multi-phase bushless DC motor driven by a pulse width modulated (PWM) amplifier, but is not limited thereto.
For explanation purposes only, the following discussion relates to the current sensing problem in multi-phase brushless DC motors and a solution for it. It will be understood, however, that the herein described invention is not limited in application to brushless DC motors.
Brushless DC motors are known. In such motors, a plurality of fixed stator windings and rotatable magnetic poles are provided. Motor rotation results by energizing each stator winding in a proper sequence. A proper sequence is one wherein a stator winding is energized when a torque producing magnetic pole is aligned with it. Generally, logic circuitry is provided which energizes the stator windings in the proper sequence. This logic circuitry requires as an input a drive signal which alternates between two states, such as a PWM signal. In order to determine the proper duty cycle of the PWM signal for a given load and a desired speed, the circuit generating the PWM signal requires a feedback signal containing information relating to both the instantaneous polarity and magnitude of the torque producing current through the stator windings.
Prior art current sensing schemes for providing feedback information to the PWM signal genration circuitry generally sense the current in each individual motor phase. The signals obtained from each motor phase are summed together to construct a waveform representative of the magnitude of the torque producing current. This is usually accomplished by placing a sense resistor in series with each motor phase, amplifying the voltage developed across each resistor and summing the amplified outputs together in the proper sequence. Summing the outputs together in the proper sequence is usually achieved by supplying the signal from each sense resistor to a separate solid state switch; the switch outputs are tied together in common and are opened and closed in the proper sequence under control of external logic. The external logic requires as an input the signal from an angular position sensor located within the motor. This sensor provides information relative to the angular position of the rotating magnetic poles.
The signal provided by the above described circuitry is representative of the instantaneous magnitude of the torque producing current, but because of the current induced by the motor's inductance, it is not necessarily representative of the correct polarity of the torque producing current.
To obtain a signal which is representative of both the instantaneous magnitude and polarity of the torque producing current, the prior art requires the use of additional circuitry. This additional circuitry is responsive to the PWM signal to pass the above described feedback signal in an inverted form when the PWM signal is in one state and in a non-inverted form when the PWM signal is in the other state.
It will be appreciated from the foregoing discussion that prior art current sensing schemes have two major drawbacks. First, in multi-phase loads, each phase requires a sense resistor and accompanying circuitry for providing a signal indicative of the magnitude of the instantaneous current through that phase. Second, circuitry is required for summing each of the phase signals in the proper sequence and for correcting the polarity of the signal obtained from the summation. In multi-phase brushless DC motors, this has required the use of an angular position sensor associated with the motor. Thus, prior art circuits for constructing a feedback signal representative of both the instantaneous magnitude and polarity of the torque producing motor current are cumbersome, expensive, and require complex logic.
It is therefore desirable to provide a circuit for generating a current signal continuously representative of both the instantaneous polarity and magnitude of the current through a load which is simple and requires few components. Such a signal can typically be used for feedback purposes but is not limited to such applications.
It is also desirable to provide a circuit for generating a current feedback signal continuously representative of both the instantaneous polarity and magnitude of the current through a multi-phase load which does not require a sense resistor for each phase and which does not require that multiple sense signals from the phases be summed together.
It is further desirable to provide a circuit for generating a current feedback signal continuously representative of both the instantaneous polarity and magnitude of the torque producing current in a multi-phase brushless DC motor which requires only a single sense resistor and does not require an angular position sensor associated with the motor to reproduce the signal and yet is simple, reliable, and inexpensive.